1. Field of the Invention
The present invention relates to an FM detector circuit.
2. Description of the Related Art
Examples of an FM detector circuit include a circuit using a Foster-Seeley method, a circuit using a ratio method, a circuit using a PLL method, a circuit using a slope detection method, and a circuit using a discriminator detection method. Japanese Unexamined Patent Application Publication No. 11-195928 (Patent Document 1) discloses an FM detector circuit using a slope detection method. According to Patent Document 1, by using impedance in a resonator, which is changed at a resonant frequency and in the vicinity thereof, frequency modulation (FM component)—amplitude modulation (AM component) conversion of a signal is performed, and furthermore, envelope detection is performed using a diode, whereby FM detection is achieved.
Japanese Unexamined Patent Application Publication No. 56-14705 (Patent Document 2) discloses an FM detector circuit using a discriminator detection method. The circuit uses impedance, which is changed at a resonant frequency and in the vicinity thereof, in a piezoelectric resonator so that amplitude modulation of an output signal is rectified by two diodes for AM detection.
For example, for a remote keyless entry receiver, an FSK signal must be demodulated. Normally, an RF signal is heterodyned into an IF signal (a superheterodyne method), and then, the IF signal is detected. However, a superheterodyne method requires a number of components such as a local oscillator, a mixer and other suitable components. To address this problem, a direct detection method in which an RF signal is directly detected without frequency conversion may be used. However, there is a high possibility that an amplifier circuit causes oscillation when an RF signal is greatly amplified for detection. In this case, an amplification factor should be suppressed. Therefore, it is desirable that a low-level RF signal is detected before being amplified.
However, the FM detector circuits according to Patent Document 1 and Patent Document 2 have problems in that since the FM detector circuits have low detection sensitivity, a detection output is low when an RF signal is at a low level.